The present invention relates to a silicon nitride sintered body having excellent high-temperature strength and oxidation resistance.
Silicon nitride sintered bodies are expected to be used as structural ceramics for gas turbine parts, etc. under severe conditions at high temperatures, because of their high strength, heat resistance, thermal shock resistance wear resistance, oxidation resistance, etc. However, since the silicon nitride itself is a material which cannot be easily sintered, various sintering aids are added to increase its density. The sintering aids tend to form glass phases of low-melting points in the boundaries of silicon nitride grains, resulting in the deterioration of high-temperature strength of the silicon nitride. Accordingly, the following attempts were proposed.
(1) Adding oxides of rare earth elements such as yttrium oxide. PA1 (2) Adding oxides of rare earth elements such as yttrium oxide together with other oxides such as aluminum oxide (for instance, Japanese Patent Publication No. 49-21091). PA1 (3) Adjusting the formulation of yttrium oxide, aluminum oxide and silicon nitride to improve an oxidation resistance at 1400.degree. C. (Japanese Patent Laid-Open No. 63-185863).
However, since these methods suffer from the following problems, they are not practical methods.
Specifically speaking, the method (1) of adding yttrium oxide solves the above problem (formation of low-melting point glass phases). In fact, since the boundaries of silicon nitride grains are bonded together by a high-viscosity glass or crystalline compositions such as Si.sub.3 N.sub.4.Y.sub.2 O.sub.3, the high-temperature strength and high-temperature creep resistance of the silicon nitride sintered body are not likely to be deteriorated. In this respect, this method is effective to some extent. However, the resulting silicon nitride sintered body shows insufficient high-temperature strength, etc. and the sintering of the silicon nitride is difficult, meaning that pressureless sintering cannot be applied.
In the method (2) where yttrium oxide and aluminum oxide are added the sintering is accelerated, and high-density, high-strength sintered bodies can be obtained even by pressureless sintering. However, since the resulting silicon nitride sintered bodies suffer from a severe decrease in high-temperature strength, dense sintered bodies having excellent high-temperature strength cannot be obtained without using special sintering methods such as hot pressing after a crystallization treatment.
With respect to sintered bodies obtained by the method (3) of adjusting the formulation of yttrium oxide, aluminum oxide and silicon nitride to improve the oxidation resistance at 1400.degree. C., the amounts of additives are extremely small. Accordingly, to achieve high density, a sintering temperature of 1850.degree. C. or higher is needed, resulting in extraordinary growth of crystal grains in the structure of the resulting sintered bodies. Therefore, although the oxidation resistance and strength at 1400.degree. C. are not lowered, they do not have sufficiently improved strength at room temperature.